Helicopter rotor blades are typically variable pitch and frequently that pitch is automatically varied during each revolution to compensate for differences in lift between the blade moving in the direction of helicopter motion and the blade moving in the opposite direction. Collective pitch is a measure of the average blade pitch and that average blade pitch is controllable by the pilot.
When a helicopter pilot causes an increase in rotor blade pitch, the blade angle of attack is increased (the blade takes a bigger bite of air) and the engine slows due to the increased load. A conventional engine governor will sense this decrease in speed and increase the fuel flow in an attempt to resume the previous speed This temporary dip in rotor speed is called rotor droop. The opposite result occurs with a decreased pitch command.
Many current control systems for turbine engines provide limited anticipation to collective pulls which allows the rotor to droop down to 95%. In addition, these systems do not provide temperature limiting (either during engine start or in flight) or torque limiting during flight or acceleration contouring. The pilot has to control the helicopter to prevent exceedences on temperature as well as torque. This adds a tremendous burden to the pilot.
An audible alarm indicative of over-stress limits including temperature, output torque and engine speed, instructing the pilot to provide the corrective action is shown in U.S. Pat. No. 4,619,110. A first low level signal is initiated when the operating parameter in question reaches a predetermined level below the allowable limit, and the audio signal is increased when the allowable limit is approached. The system automatically limits fuel flow to avoid over-stressing the engine, but this limiting may be over-ridden under emergency conditions. Fuel flow limiting is accomplished by bleeding some air from a pneumatic fuel controller. Water and/or additional fuel may also be automatically injected into the engine fuel flow. This patented scheme utilizes pneumatic control of one fuel valve for all fuel flow control. Flowing pneumatic systems are limited in terms of reliability because of the inherent issues associated with flowing air laden with the products of combustion through computational control circuits. Another limitation of such control systems is the failure to offer redundancy regarding power turbine governing. Subsequently system safety could be improved in this area.
New full authority digital electronic control (FADEC) systems that have been developed in recent years offer solutions to these problems but these systems are high cost and are only offered on new expensive model helicopters.
In our aforementioned copending application, an engine over temperature avoidance technique operable only during engine start-up monitors engine temperature and diminishes fuel flow to the engine when that monitored temperature exceeds a threshold temperature. While this system admirably performs its intended function of avoiding thermal stress to the engine during start-up, its function ceases upon the engine reaching normal idle speed.